you used this stuff, correct? not some ebay, direct from china "W3" tape?
because i installed this tiny gag spoiler with 2 tiny little pieces of that tape, and it stayed on for 2 years outside through rain, sleet, snow, and ice, until a youth group charity car wash baseball-batted it off a few weeks ago.
my first suspicion is that you didn't get the right 3m tape. don't buy it online, buy it from a local auto parts store only. it's the best way to know what you're getting.